This invention relates to the field of testing of mass-produced semiconductor device. More particularly the invention relates to measuring a characteristic of a test device on a semiconductor chip to determine the performance of other devices on the chip.
Process monitor circuits are used in semiconductor manufacturing processes to monitor, at various points during production, whether the processes are producing semiconductor devices on a very large scale integrated (VLSI) circuit chip that perform according to specification. Monitoring is typically done by measuring the characteristics of the monitor circuit that are related to the process conditions used to fabricate the integrated circuits on a monolithic wafer. The process monitor circuits may be placed within every one of the production circuits, or dice, produced. In this manner, the process monitor circuit in each packaged device can be used as a data point to determine process characteristics.
One type of process monitor circuit is an oscillator circuit manufactured as part of the production integrated circuit wafer. When the oscillator circuit is excited, its oscillation frequency gives an indication of the performance of transistor devices in the oscillator circuit, and of the performance of other devices on the wafer.
Most VLSI circuit testers are not capable of monitoring an oscillator frequency directly. Thus, to measure oscillator frequency, integrated circuit manufacturers have had to resort to building expensive mixed-signal testers implemented in a xe2x80x9crack-and-stackxe2x80x9d test equipment format.
What is needed, therefore, is a testing circuit on the production chip that provides an oscillator frequency output signal that may be monitored by a standard VLSI circuit tester.
The above and other needs are met by a testing circuit formed on a semiconductor wafer that provides an output signal indicative of at least one operational characteristic of semiconductor devices formed on the wafer. The output signal provided by the testing circuit is compatible for monitoring using an integrated circuit tester. The testing circuit includes an oscillator, a counter, and a shift register, all formed on the semiconductor wafer. The oscillator produces oscillator pulses at an oscillator frequency when the oscillator is energized, where the oscillator frequency is indicative of at least one operational characteristic of the semiconductor device. The counter receives and counts the oscillator pulses from the oscillator, and produces a count corresponding to the number of oscillator pulses received by the counter during a predetermined length of time. The shift register receives the count from the counter as a digital data word comprising multiple bits.
In another aspect, the invention provides a system for determining at least one operational characteristic of semiconductor devices formed on a semiconductor wafer. The system includes an oscillator formed on the semiconductor wafer for producing oscillator pulses at an oscillator frequency when the oscillator is enabled, where the oscillator frequency is indicative of at least one operational characteristic of the semiconductor devices. The system also includes a counter formed on the semiconductor wafer for receiving and counting the oscillator pulses from the oscillator, and for producing a count corresponding to the number of oscillator pulses received by the counter during a predetermined length of time. A shift register formed on the semiconductor wafer receives the count from the counter as a digital data word comprising multiple bits. The system further includes a tester for receiving the count from the shift register, for determining the oscillator frequency based on the count and the predetermined time period, and for determining the at least one operational characteristic of the semiconductor devices based on the oscillator frequency.
In a most preferred embodiment, the tester is operable to reset the counter, enable the oscillator to begin producing the oscillator pulses, disable the oscillator after a predetermined time period, shift out the multiple bits of the count from the shift register, and determine the oscillator frequency based upon the multiple bits of the count and the predetermined time period.
In yet another aspect, the invention provides a method for testing a semiconductor device to determine at least one operational characteristic of the semiconductor device. The method includes resetting an N-bit counter, enabling an oscillator on the semiconductor device to produce oscillator pulses at an oscillator frequency during a predetermined time period, and counting the oscillator pulses with the counter during the predetermined time period to determine a pulse count. The method also includes producing a digital data word comprising N number of bits corresponding to the pulse count, and shifting the N number of bits of the digital data word into a shift register having a most significant bit and a least significant bit. The N number of bits of the digital data word are shifted out of the shift register, and are manipulated to determine a count value. The method further includes determining an oscillator frequency value by dividing the count value by a time value corresponding to the predetermined time period.